1. Field of the Invention
The present invention relates to a method and apparatus for testing a magnetic bubble memory, and more particularly to a testing method and apparatus which utilizes a test information pattern written into a storage area of a magnetic bubble memory chip.
2. Description of the Prior Art
It has been well known that in a single crystal thin film such as a certain type of rare earth garnet or ortho-ferrite, when an easy axis of magnetization is formed perpendicular to a plane of the film, circular magnetic domains (referred to as magnetic bubble domains) are generated by the application of a magnetic field in the direction of the easy axis of magnetization (referred to as a bias magnetic field), and that a magnetic bubble memory can be constructed by providing means for moving the magnetic bubble domains in the plane of the film. The magnetic bubble memory has been recently researched for actual implementation. In general, in a presently available magnetic bubble memory, the diameter of the magnetic bubble domain is equal to 1-5 .mu.m and total of 16,000-1,000,000 magnetic bubble domains are held on a 3 to 6 mm square chip. Those magnetic bubble domains are positioned in stationary fashion by a geometrically arranged pattern of soft magnetic material thin film such as permalloy and they can be regularly transferred between columns of the pattern of the soft magnetic thin film by externally applying a rotating magnetic field in the plane of the film by the moving means. FIG. 1 shows a diagram of what is called a serial memory organization in which a closed loop 1 is constructed by the pattern of the soft magnetic thin films for regularly transferring the magnetic bubble domains. FIG. 2 shows another organization which is referred to as a major/minor loop memory organization in which a plurality of closed minor loops 2 are constructed by the patterns of the soft magnetic thin films and a closed major loop 3 for performing a common function is arranged adjacent to the closed loops 2.
The latter system comprises the minor loops 2 being used to store information therein and the major loop 3 being used to read and write the information. Satisfactory performance for a memory device requires that it be able to conduct write, store, and read operations of information without erroneous operation, to produce the characteristic of high precision at high speed. Such memory performance is directly or indirectly influenced by physical phenomena dependent on memory function, material to be used and the method of fabricating the memory device.
Since the magnetic bubble domains in a storage area carry an information "1" or "0" for each bubble domain, and are arranged at a given interval as closely as possible to each other in order to achieve a magnetic bubble domain density, which is called a "linked-bubble pattern," or "linked pattern," magnetic interactions are created among the magnetic bubble domains, as a matter of course although they are limited to a practical extent. In certain cases, such interactions impede proper transfer of the magnetic bubble domains.
This is one of the important factors which influence a memory characteristic such as that noted above. In order to evaluate such a memory characteristic, it is important to consider the dependency of a bias margin characteristic on the magnetic bubble domain arrangement, that is, the difference of influence of interaction between, at an upper limit, when the bias magnetic field applied to form the magnetic bubble domains (referred to as an upper limit of bias magnetic field) is increased and, at a lower limit, (referred to a lower limit of bias magnetic field) when the bias magnetic field is decreased, and also to consider the dependency on the linked pattern previously discussed by which the magnetic bubble domains are arranged in the storage area. The worst condition of the linked pattern of the magnetic bubble domains, that is, the condition which results in the largest magnetic interaction is especially important. In other words, the memory characteristic under the worst condition is most important to consider.
In testing such memory characteristics, a simple pattern such as a mode in which the magnetic bubble domains are arranged at all locations (referred to as an all "1's" pattern) or a mode in which no magnetic bubble domain is arranged at all locations (referred to as an all "0's" pattern) has been used, but as mentioned above, none of them individually realizes the worst condition for both upper and lower limits of bias magnetic field when taking the interaction among the magnetic bubble domains into consideration. Namely, the test has a drawback that it cannot exactly evaluate the memory characteristic.